It is common to drive a series string of LEDs using a DC/DC switch mode power supply converter. One end of the LED string may be connected to a reference source (e.g., ground or the input voltage), or both ends of the string may be floating, since only the regulated current through the string is relevant to light output. The converter can be a step up (boost) or step down (buck) converter. There are other types of converters (buck-boost) using four switches that boost or buck the input voltage as needed. The additional switches add cost and size, and the transition between the buck and boost modes can create noise. Such buck-boost converters do not have both low input voltage ripple and low output voltage ripple, so generate higher EMI than desirable.
What is needed is an efficient converter for a floating load, such as a string of LEDs, where only two switches are required, and where the configuration inherently smoothes both input and output current ripple for low EMI. The converter should output a voltage across the floating load that can be either greater than or less than the input voltage while regulating the LED string current. The switch controller may be an off-the-shelf buck controller IC due to features offered by buck controllers that are not available with boost controllers or buck-boost controllers. Using such an off-the-shelf buck controller will also greatly reduce the cost of implementation of the converter.